U.S. Pat. No. 7,244,867 describes a process for converting lower molecular weight alkanes, including methane, natural gas or ethane, propane, etc., into higher molecular weight hydrocarbons, including aromatics, by bromination to form alkyl bromides and hydrobromic acid which are then reacted over a crystalline alumino-silicate catalyst to form the higher molecular weight hydrocarbons and hydrobromic acid. Hydrobromic acid is recovered by contacting the reaction product stream with water and then converted to bromine for recycle. The higher molecular weight hydrocarbons are recovered.
In a process for producing aromatic hydrocarbons such as benzene, toluene and/or xylenes (BTX) and/or C4+ nonaromatic hydrocarbons by bromination of methane to produce monobromomethane, followed by coupling of the monobromomethane to produce the desired hydrocarbons, the coupling reactor produces hydrogen bromide (HBr), and unintended amounts of methane, light ends (C2-3 alkanes and alkenes) and heavy ends (C9+ aromatic hydrocarbons). The basic process concept may include recycle of the light ends, possibly to a separate bromination reactor, and the use of methane and heavy ends as fuel.
Methane present in natural gas may be brominated by a high temperature reaction with elemental bromine to produce brominated products consisting primarily of methyl bromide and hydrogen bromide (HBr). In separation steps, HBr may be recovered for bromine regeneration, poly bromides recovered for recycle, and methyl bromide recovered. In the second reaction step, methyl bromide may be passed over a catalyst at high temperature to carry out a coupling reaction that yields a product mixture containing aromatic and aliphatic compounds and HBr. Alternately, in the second reaction step the entire stream from the bromination step is employed. Product separation steps are employed to recover the HBr byproduct and to separate the organic products into light hydrocarbons, C4+ non-aromatic hydrocarbons, aromatics and heavy ends. Brominated organic compounds are present as undesirable impurities in the organic product streams. While organic bromides (even methyl bromides) can be separated from C1-3 hydrocarbons simply by distillation, these impurities are very difficult to remove from aromatic hydrocarbons and C4+ non-aromatic hydrocarbons by conventional techniques such as distillation or selective absorption.
Very stringent product specifications must be met with respect to halogenated impurities in the finished product. Undesirable refractory (difficult to remove) brominated organic impurities are often present in the hydrocarbon product streams.
It can be seen that it would be advantageous to provide an integrated process concept wherein undesirable brominated organic impurities could be removed or converted into hydrocarbons, possibly for recycle, to greatly reduce the total amount of brominated organic impurities that must be disposed of. The present invention provides such an integrated process.